1. Field of the Invention
The present invention relates generally to a pneumatic signal generator for testing sensors and instrumentation, and more particularly to a ground-based pneumatic generator system for simulating operating pressure conditions of an aircraft.
2. Description of the Related Art
Pneumatic function generator (PFG) systems are used to control the pressure of a particular space by outputting a desired pressure and/or regulating a rate of pressure change per unit time. Such a system can be used, for example, to calibrate or test pressure sensors. FIG. 1 is a block diagram of a prior art PFG system 100, which includes a pressure servo valve 102, a control signal generator 104, an isolation valve 106, a sensor 108, and an output port 110. The pressure servo valve 102 directly controls the amount of pressure or vacuum being directed to the output port 110. Using the sensor 108, the PFG system 100 provides closed-loop control of the pressure and rate of change of pressure as air or other gases are being output from the output port 110. In particular, the sensor 108 measures the pressure and/or the rate of change of pressure and provides data to the control signal generator 104, which outputs a control signal to adjust the pressure servo valve 102. The control signal may output a positive value to increase the pressure or a negative value to decrease the pressure. One drawback of conventional PFG systems is the non-linear changes to the pressure being output from the output port 110.
The PFG system 100 may be used to test the seal integrity of external pneumatics by implementing what is referred to as a “leak check”. A typical leak check involves driving the external pneumatic to a specific pressure and then closing the isolation valve 106 to isolate the pressure servo valve 102 from an external load 112, which is connected to the output port 110. The sensor 108 is connected to the output port 110 to monitor the pressure in the external pneumatic and to detect changes in the pressure that might be indicative of leaks.
The arrangement shown in FIG. 1, however, causes the pressure servo valve 102 to be part of an open-loop configuration when a leak check is performed. This means that given a sufficient amount of time, the pressure on the pressure servo valve side will eventually rail, either to maximum pressure or maximum vacuum. This can result in potentially hazardous conditions if a significant pressure delta appears across the isolation valve 106. When the isolation valve 106 is eventually reopened, the pneumatic pressure rapidly seeks to equalize itself. This rapid equalization of pressure may result in a hammer effect causing undesirable stress or damage to the sensor 108 or the external load 112. In addition, conventional PFG systems are susceptible to the effects of temperature, aging, component drift, and wear and tear of the pressure servo valve components, which ultimately degrade the performance of the PFG system 100.
Thus, it should be appreciated that there is a need for a pneumatic generator system that overcomes the drawbacks of conventional PFG systems. The present invention fulfills this need as well as others.